A modifier plug-in must implement the method Modifier::InputType() to inform 3ds Max which kinds of input object it requires in order to perform a modification. Most modifiers will ask for deformable objects. See Deformable Objects for more information.
Most objects need to be able convert themselves to TriObjects and TriObjects are deformable, so it is sufficient to require triObjectClassID. Modifier::InputType() can return any Class_ID such as: triObjectClassID, defObjectClassID, patchObjectClassID.
For simple modifiers SimpleMod implements the Modifier::InputType() method and returns defObjectClassID to indicate that it can handle any deformable objects.